1. Field of the Invention
The present invention relates to vehicle control systems for reducing the speed of a vehicle to improve vehicle dynamic conditions through the application of a vehicle brake actuation.
2. Description of the Related Art
In a variety of vehicle control systems it is advantageous to reduce the speed of the vehicle to improve the vehicle dynamic condition. This is primarily accomplished through the application of the vehicle brakes by hydraulic actuators which include pumps and solenoid valves. The fluid flow rate of the braking system is limited by the motor design, voltage, pump capacity, internal resistances and other braking system parameters. The fluid flow rate of pressurized brake fluid to the vehicle brake actuators provides one limit to the rate at which the speed of the vehicle is reduced. Under certain conditions such as ABS, roll mitigation, or other vehicle stability dynamic controls it is critical to achieve an instantaneous high level of deceleration for a vehicle. During conventional braking, pressurized brake fluid is supplied to all four brake actuators of the vehicle for a respective braking system having similar braking circuits and components and is equally distributed. As a result, the pumping of fluid to build pressure within each of the vehicle brake actuators during the initial braking stages will take longer than a brake circuit of a same volume but having fewer brake actuators.
Most pressure-volume relationships for brake calipers requires more fluid at lower pressures during initial application of applying a braking force on each of the brake actuators followed by a linear increase of fluid volume up to maximum pressure. This initial volume requirement is mostly due to knockback of the actuator pistons and seal compliance. This can be represented as a braking force to volume relationship. As a result, a large fluid volume is initially required at high flow rates of the brake fluid within a brake circuit. To avoid low braking forces during initial braking conditions, an increased pump flow rate must be obtained in each of the hydraulic brake circuits to increase the flow of brake fluid to each brake actuator. This results in high system cost.